When would you choose SQL vs NoSQL databases?

SQL databases (PostgreSQL, MySQL) for: structured data with relationships, complex queries and joins, ACID transactions, data integrity critical. NoSQL databases (MongoDB, Cassandra, Redis) for: flexible/evolving schemas, high write throughput, horizontal scaling, specific data models (document, key-value, graph). Consider: consistency requirements (CAP theorem), query patterns, scaling needs, team expertise. Many systems use both (polyglot persistence) for different use cases.